This section provides background information related to the present disclosure which is not necessarily prior art.
U.S. Pat. No. 7,397,169 by Nersesse Nersessian, Gregory P. Carman, and Harry B. Radousky for energy harvesting using a thermoelectric material includes the state of technology information reproduced below.
Waste heat is always generated whenever work is done. Harvesting such waste heat can increase the efficiency of engines, be used to power numerous devices (eliminating the need for auxiliary power sources), and in general, significantly reduce power requirements. Various methods have been used to try and harvest such waste heat and mechanical energy. For waste heat the most important of which is through thermoelectric materials. In order to efficiently convert waste heat to usable electrical energy, thermoelectric materials generally requires a large Seebeck coefficient having a “figure of merit” or Z, defined as,
  Z  =                    s        2            ⁢      σ        k  where S is the thermoelectric power, i.e., the Seebeck coefficient, σ is the electrical conductivity (=1/ρ), σ is the electrical resistivity and k is the thermal conductivity and. A dimensionless number ZT is often used as a figure of merit for the TE material. For conventional materials the ZT<1 at T=300 K. The higher the ZT, the more efficient the TE. ρ is the electrical resistivity, and K is the thermal conductivity. The Seebeck coefficient is further defined as the ratio of the open-circuit voltage to the temperature difference (δT) between the hot and cold junctions of a circuit exhibiting the Seebeck effect, or S=V/(δT). Therefore, in searching for a good thermoelectric material, materials with large values of S, and low values of p and k are beneficial.
However, current state of the art thermoelectric materials utilized to harvest waste heat and convert such heat to a useful energy, for example, devices that use a combination of n-type and p-type materials, generally have Z values which are prohibitively low for harvesting energy from low quality waste heat, where δT is on the order of 100° C. or less.
Additional background information on thermoelectric devices is described in U.S. Pat. No. 5,550,387, entitled “Superlattice Quantum Well Material,” issued Aug. 27, 1996 to Elsner et al. (hereinafter the “Elsner '387 patent”). The Elsner '387 patent involves thermoelectric elements having a very large number of alternating layers of semiconductor material. The alternating layers all have the same crystalline structure. This makes the vapor deposition process easy because the exact ratio of the materials in the layers is not critical. The Elsner '387 patent demonstrates that materials produced in accordance with the teachings thereof provide figures of merit more than six times that of prior art thermoelectric materials. A preferred embodiment discussed in the Elsner '387 patent is a superlattice of Si, as a barrier material, and SiGe, as a conducting material. Both of these materials have the same cubic structure. Another preferred embodiment which is discussed is a superlattice of B—C alloys, the layers of which would be different stoichiometric forms of B—C but in all cases the crystalline structure would be alpha rhombohedral. In a described preferred embodiment the layers are grown under conditions as to cause them to be strained at their operating temperature range in order to improve the thermoelectric properties.
Background information on an energy harvesting system is described and claimed in U.S. Pat. No. 2004/0238022 A1, entitled “Thermoelectric Power From Environmental Temperature Cycles,” issued Dec. 2, 2004 to Hiller et al. (hereinafter the “Hiller '022 application”). The Hiller '022 application involves an electric generator system for producing electric power from the environmental temperature changes such as occur during a normal summer day on Earth or Mars. In a described preferred embodiment a phase-change mass is provided which partially or completely freezes during the relatively cold part of a cycle and partially or completely melts during the relatively hot part of the cycle. A thermoelectric module is positioned between the phase-change mass and the environment. The temperature of the phase-change mass remains relatively constant throughout the cycle. During the hot part of the cycle heat flows from the environment through the thermoelectric module into the phase change mass generating electric power which is stored in an electric power storage device such as a capacitor or battery. During the cold part of the cycle heat flows from the phase change mass back through the module and out to the environment also generating electric power that also is similarly stored. An electric circuit is provided with appropriate diodes to switch the direction of the current between the hot and cold parts of the cycle. A preferred phase change mass is a solution of water and ammonia that has freeze points between about 270° K to about 145° K depending on the water ammonia ratio. It is further described that, preferably, a finned unit is provided to efficiently transfer heat from a module surface to the environment.
Various attempts at using nanowires for mechanical energy harvesting have been made, with varying degrees of success. Information on mechanical energy harvesting may be found at: Chen X, Xu S, Yao N, Shi Y. “1.6 V Nanogenerator For Mechanical Energy Harvesting Using PZT Nanofibers,” Nano Letters. 2010 May 25; 10(6), pp. 2133-7; Qin, Y., Wang, X. and Wang, Z. L. “Microfibre-Nanowire Hybrid Structure For Energy Scavenging. Nature, 451(7180), 2008 Feb. 14, pp. 809-813; Wang Z L, Song J. “Piezoelectric Nanogenerators Based On Zinc Oxide Nanowire Arrays,” Science. 2006 Apr. 14, 312(5771), pp. 242-6; Xu C, Wang Z L. “Compact Hybrid Cell Based On A Convoluted Nanowire Structure For Harvesting Solar And Mechanical Energy,” Advanced Materials, 2011 Feb. 15, 23(7), pp. 873-7; Xu S, Qin Y, Xu C, Wei Y, Yang R, Wang Z L. “Self-powered nanowire devices,” Nature Nanotechnology, 2010 May 1, 5(5), pp. 366-73; Xu S, Hansen B J, Wang Z L. “Piezoelectric-Nanowire-Enabled Power Source For Driving Wireless Microelectronics,” Nature Communications. 2010 Oct. 19, pp. 1-5; Ko S H, Lee D, Kang H W, Nam K H, Yeo J Y, Hong S J, Grigoropoulos C P, Sung H J. “Nanoforest Of Hydrothermally Grown Hierarchical ZnO Nanowires For A High Efficiency Dye-Sensitized Solar Cell,” Nano Letters. 2011 Jan. 5; 11(2), pp. 666-71; Song J, Zhou J, Wang Z L. “Piezoelectric And Semiconducting Coupled Power Generating Process Of A Single ZnO Belt/Wire. A Technology For Harvesting Electricity From The Environment,” Nano Letters. 2006 Aug. 9, 6(8), pp. 1656-62; Zhu G, Yang R, Wang S, Wang Z L. “Flexible High-Output Nanogenerator Based On Lateral ZnO Nanowire Array,” Nano Letters. 2010 Jul. 21, 10(8), pp. 3151-5; Yang Y, Zhang H, Zhu G, Lee S, Lin Z H, Wang Z L. “Flexible Hybrid Energy Cell For Simultaneously Harvesting Thermal, Mechanical, And Solar Energie,”. ACS Nano. 2012 Dec. 5, 7(1), pp. 785-90.
In view of the foregoing, it will be appreciated that there presently is significant interest in systems and/or methods which enable energy to be harvested. The successful harvesting of energy could be used for a wide variety of beneficial purposes, for example to power sensors, photovoltaic cells, and other electronic components. However, the complexity and drawbacks of present day systems makes many present day systems either too expensive or too inefficient for many energy harvesting applications.